Oil film imaging

ABSTRACT

A METHOD OF FORMING AN IMAGE WHEREIN A PHOTOCONDUCTIVE INSULATING LAYER HAVING A THIN OIL OR INTERFERENCE FILM THEREON AND SEPARATED FROM AN ELECTRODE BY A SMALL GAP IS SELECTIVELY ILLUMINATED BY A PATTERN OF LIGHT WHILE AN ELECTRIC FIELD IS ESTABLISHED BETWEEN THE PHOTOCONDUCTIVE LAYER AND THE ELECTRODE CAUSING LIQUID TRANSFER IN IMAGEWISE CONFIGURATION ONTO THE ELECTRODE. DURING THIS PROCESS AN IMAGE IS ALSO FORMED ON THE PHOTOCONDUCTIVE INSULATING LAYER. ALTERNATIVELY, A THIN OIL FILM MAY BE APPLIED SOLELY TO THE ELECTRODE OR TO BOTH THE PHOTOCONDUCTIVE INSULATING LAYER AND THE ELECTRODE.

Jan. 2, 1973 w. l.; GOFFE y 3,708,287

OIL FILM IMAGING Filed April 19. 1971 /NvE/vro/P F/G 2 WILLIAM L .GoFf-'E 24AM/uu d A TTORNEY United States Patent O Int. Cl. G03g 13/22 U.S. Cl. 96-1.3 12 Claims ABSTRACT OF THE DISCLOSURE A method of forming an image wherein a photoconductive insulating layer having a thin oil or interference film thereon and separated from an electrode by a small gap is selectively illuminated by a pattern of light while an electric field is established between the photoconductive layer and the electrode causing liquid transfer in imagewise configuration onto the electrode. During this process an image is also formed on the photoconductive insulating layer. Alternately, a thin oil film may be applied solely to the electrode or to both the photoconductive insulating layer and the electrode.

This application is a continuation-in-part application of Ser. No. 571,343 filed Aug. 9, 1966, now abandoned, and relates to a novel means and method for image reproduction in which neither chemical reactions nor particulate materials are necessarily involved. More particularly, the invention forms an image by migration of tiny liquid droplets across a gap from one electrode to another under the influence of an electric field.

BACKGROUND O-F THE INVENTION In conventional electrography an electrostatic latent image is formed by the combined action of an electric field and a pattern of light on a photoconductive insulating layer. The latent image thus formed is rendered visible by the deposition of finely divided particles thereon. These finely divided particles are selected to have an initial charge so that they will be attracted to the latent electrostatic image. U.S. Pat. No. 2,825,814 to Walkup is an example of a conventional electrographic imaging technique.

In accordance with the present invention, a novel method of electrographic imaging is provided wherein a photoconductive insulating layer having a thin liquid oil film also called an interference film thereon is separated from an electrode by a narrow gap. When the photoconductive insulating layer is selectively illuminated by a pattern of light and an electric field is established between the photoconductive layer and the electrode, liquid transfer in imagewise configuration is formed on the electrode. That is, tiny droplets are transferred to the electrode and form a pattern thereon identical to the light pattern which selectively illuminated the photoconductive insulating layer. The oil or interference film employed in the invention is preferably of the type described in U.S. Pat. No. 3,196,010 to Goffe et al. and US. 3,394,002 to Bickmore.

Accordingly, it is an object of this invention to provide a method of forming an image by the transfer of tiny liquid droplets under the infiuence of an electric field.

It is a further object of this invention to provide a method of forming an image by selectively illuminating a photoconductive insulating layer having an oil or interference film thereon so that tiny droplets are transferred in imagewise configuration to an electrode placed in very close proximity to the photoconductive insulating layer.

It is yet a further object of this invention to provide a ice method of forming an image by liquid transfer wherein there is a narrow gap between the photoconductive layer holding the thin film and the electrode.

It is still another object of this invention to provide a method of forming an image by liquid transfer wherein a thin oil or interference film is formed on both the photoconductive insulating layer and the electrode.

Another object of this invention is to provide a method of erasing the image formed by liquid transfer by subjecting the image to a source of light.

lBRIEF DESCRIPTION OF THE INVENTION In a first embodiment of the invention a photoconductive insulating layer is placed upon a first transparent electrode. A non-gap-bridging, non-volatile insulating oil or interference film is then placed upon the photoconductive layer. A second electrode is then positioned substantially parallel to the photoconductive insulating layer leaving a gap of less than .001 inch. Next, means are provided to project a pattern of light on the photoconductive layer. Thereafter, a source of electrical power is connected across the first and second electrodes causing migration of tiny liquid droplets in imagewise configuration across the gap to the second eletcrode. In a second embodiment of the invention a dielectric layer is placed upon the flat surface of the second electrode facing the photoconductive insulating layer to increase resolution. vIn a third embodiment of the invention a liquid interference film is positioned on both the photoconductive layer and the second electrode. The image formed by the above steps may be developed by dusting the formed image with a finely divided powder. Alternately, the formed image may be developed by transferring the image to a support treated to react chemically with the formed image.

The nature of the present invention having been set forth, there will now be presented a more detailed description in illustration but not limitation of the invention in the following specification and drawings in which:

FIG. 1 is a schematic cross-sectional view of the apparatus used in carrying out the invention, and

FIG. 2 illustrates a method of precharging the photoconductive insulating layer.

Turning now to FIG. 1, two plate-like electrodes 10 and 12 are employed, each having at least one fiat surface. The fiat surface of electrode 12 is covered with a thin layer 14 having a thickness of about .O02 inch of a photoconductive insulating material. A photoconductor is a material that is sufficiently resistive n darkness to be classified as a true electrical insulator but that becomes relatively more conductive upon exposure to radiation such as light. A vacuum evaporated layer of vitreous selenium is the most common material of this classification and is a preferred material for use in the invention, but other materials with simi-lar properties are known to the art and may be employed. An example of one of these that enjoys substantial commercial use is a dispersion of zinc oxide in a resin binder.

In one embodiment of the invention, electrode 10 is covered with a thin layer of insulating material 11 having a thickness of a few microns. Either photoconductive insulating layer 14 or the facing surface of electrode 10 or both of them, in that order of preference, is covered with a thin continuous layer of oil or liquid interference film 16. r[he term oil or liquid interference film as ernployed in this specification relates to any liquid material that has a sufficiently low vapor pressure of about 10-3 mm. to form stable thin films and that offers high resistance to current flow sufficient to maintain potential gradients along the surface of the filtri. The temperature of the thin film must not have a deleterious effect on the photoconductive layer. Otherwise, there is no criticality of temperature conditions. A resistivity of about 109 ohm-cm. or the resistivity of thin films discussed in Goffe et al., have been found to be operable for purposes of the invention. The films employed in the present invention are so thin that the resistivity requirement is met by mostliquids. lEster type iiuids sold for use as vacuum diffusion pump oils under such names as Octoil (dioctal phthalate) made by Bendix Corporation, Vacuum Division, further described at page 648 of the 1937 edition of Hackhs Chemical Dictionary, and Narcoil (dinonyl phthalate) made by National Research Corporation further described at page 143 of the 1953 edition of the Handbook of Material Trade Names by O. T. Zimmerman and I. Lavine are particularly suitable for use in the invention. Oil or interference film 16 must be very thin, preferably less than about 1 or 2 microns. Films of this thickness range can be recognized by the interference fringes or patterns which they produce.

The thin oil or interference lm of this invention is transparent and has a minimum thickness necessary to show interference between the light from its two reflecting surfaces since this interference is a means by which images are made visible in the film. The oil or interference film has optical properties akin to an oil film on water that exhibits bands of color. Light is reflected from each of the surfaces of the interference film and the two reflected beams will either reinforce each other or subtract from each other depending on the phase difference between the light reflected from the top and bottom surfaces. This effect is well recognized in the field of optics. Interference is first evident at the quarter wavelengths of the shortest wavelength of visible light where films have a thickness of about 120 millimicrons. For quarter wavelength films, monochromatic light from the second or bottom surface travels a total of a half wavelength further than the light reflected at the top or first surface. This means that the reflected waveforms are 180 out of phase and results in destructive interference. The resultant amplitude is then the difference of the reiiected light from each surface. To achieve complete cancellation of the refiected waves from each surface the reflectivity of the second surface will have to be somewhat greater than the reflectivity of the first surface since the light traveling the longer distance undergoes losses at each of its passages through the first surface. It is not necessary for the purposes of this invention that complete cancellation occur.

Fuller details concerning the interference characteristics of the thin film used in this invention may be had by referring to U.S. Pat. No. 3,196,010 to Golfe et al. cited above. In any event, the preferred range of thickness of the oil or interference film is in the general range of 80 to 125 millimicrons. The contact angle of the oil or interference film should be such that it wets the surface on which it lies. In other words, the oil or interference film have a wettability below the critical value of selenium, or other surface on which it resides. The viscosity of the film should be in the range of the viscosity of Narcoil which has a viscosity of 55.5 centistoke (100 F.). However, viscosities in the range of thin films utilized in Goffe et al. are also suitable.

A simple way to form the required oil or interference film 16 is to separate electrodes 10 and 12 and rub a dilute mixture of the selected oil in a volatile solvent such as acetone, onto the surfaces with a piece of cotton wool. The resulting liquid lilrn will be much thicker than desired but the solvent will rapidly evaporate and reduce the film thickness to the desired range. Further details for forming the oil or interference film are disclosed in U.S. Pat. No. 3,196,010 to Golfe et al. and are incorporated herein by reference to that patent.

After the oil or `liquid interference film 16 has been formed, electrodes and 12 are positioned as illustrated. The gap 18 between electrode 10 and photoconductive insulating layer 14 should be on the order of 2 to 7 microns and preferably not more than about .001 inch. This spacing can be achieved in a practical manner by using thin spacers, not shown, between the electrodes 10 and 12 or between the electrode 10 and the outer edge of photoconductive insulating layer 14. Strips of .00025 inch Mylar polyester film are effective spacers for use in the latter position. A D.C. power supply 20 and switch 21 is connected between electrodes 10 and 12 and supplies a voltage in the range of SOO-1200 volts and generally about 600 volts in order to create an electric fie-ld in gap 18.

The only further step required to form an image is to selectively illuminate photoconductive insulating layer 14 with an image pattern of light. To accomplish this, either electrode 10 or electrode 12 should be optically transparent. For this purpose, the electrodes may comprise sheets of transparent material with a thin transparent surface coating of an electrically conductive material. Glass coated with a thin transparent conductive layer of tin oxide is a preferred material known in the art and is available commercially under various trade names. The electrodes may also be constructed by applying very thin metallic films or films of copper iodide to sheets of transparent material such as glass or plastics.

I have found that with the apparatus of IFIG. 1 less exposure is required to satisfactorily illuminate photoconductive insulating layer 14 through electrode 12, and accordingly there is shown an original subject 22 illuminated by lamps 24 and which is focused by lens 26 through transparent electrode 12 onto the back of photoconductive insulating layer 14. Contact exposure may also be employed. However, I have also successfully carried out my invention by exposing layer 14 through a transparent electrode 10. When layer 14 is selectively illuminated by an image pattern of light, a visible image appears within a few seconds at most on both the photoconductive insulating layer 14 and the electrode 10.

The mechanism of image formation is not fully understood, but it appears that microscopic droplets of oil break off from the oil films and are tranpsorted across the gap 18 under the control of the pattern of resistivity in layer 14 as caused by the selective illumination from subject 22. If the oil films are thicker than indicated, large globules will transfer and form an image pattern of varying film thickness. If gap 18 is thicker than indicated, there will be a loss in resolution. For similar reasons insulating layer 11, if used, should have a thickness in the order of microns.

If layer 14 is illuminated through electrode 12, as shown, and electrode 10 is also transparent, then a visible image is immediately viewable by weak illumination through electrode 10 because photoconductive insulating layer 14 appears less sensitive to illumination through electrode 10 than through electrode 12. If, however, a bright light is shown through electrode 12 or if subject 22 is replaced by a sheet of white paper, for example, the image will be erased from layer 14 and electrode 10 and a new image can be formed later. When the original exposure conditions are restored the image will reappear. When the illumination is stopped, switch 21 disconnected and electrodes 10 and 12 are separated, a reasonably stable image will remain on the electrodes because extremely thin oil or interference lms such as I employ are not readily selfleveling. Once power supply 20 is disconnected or electrodes 10 and 12 are separated, ambient illumination will have no effect on the images. The images are readily visible as an interference pattern and may be projected.

Since the images are in the form of a thin liquid interference film they may be transferred by rolling to a support such as a sheet of paper or the like. The `transferred image is not itself visible unless it has been treated with a first chemical which reacts with a second chemical in the paper or support to form a color reaction. Color reaction imaging per se is well known. The invisible transferred image can also be made visible by dusting the sheet with a finely divided powder which will selectively adhere to the areas having the greatest amount of oil. Such a development technique may also be employed to enhance the visibility of the oil image directly on electrode or layer 14, but the dusted layer or electrode cannot be reused until the powder is washed off.

FIG. 2 illustrates a method of precharging photoconductive insulating layer 14. Layer 14 is passed beneath a corona generating devices 30 which is maintained at a potential of several thousand volts relative to electrode 12 by a high voltage D.C. power supply 32. If this operation is carried out in darkness, photoconductive insulating layer 14 can lbe charged to a high surface potential, illustratively about 600 volts positive for selenium layers. The charging operation may be carried out after an oil or interference film 16 has been formed on layer 14. After charging, the apparatus may be assembled in the manner shown in FIG. 1, except that power supply 20 can be of much lower voltage or even be replaced by a simple connecting wire, since the surface potential on layer 14 will create the necessary electric field in gap 18.

Using the apparatus of FIG. l, the application of voltage from power supply 20 for a time as short as 1/2 second or less is sufficient to form an image. If no insulating layer 11 is present, it appears that tiny droplets of oil are carried back and forth across gap 18 in illuminated areas, causing a redistribution of oil film 16 as originally laid down. It is postulated that there is an air breakdown occurring within the gap and that ion deposition on the oil film may locally destroy the surface tension and permit droplet formation in the presence of the electric field, which is strongest in illuminated areas. When the exposure intensity is increased to very high levels a reversal of image polarity is observed.

When an insulating layer 11, such as Staybelite rosin ester, is employed on electrode 10, the apparatus is more sensitive to light and tiny droplets of oil are transported across the gap in non-illuminated areas, instead of in illuminated areas when insulating film '11 is absent. It is believed that charge transfer takes place to the insulating layer 11 in areas of exposure and that the potenti-al -build up on insulating layer 11 reduces the field in the gap and prevents transport of oil across the gap. If the apparatus of FIG. l is tested without oil or interference film 16 but with insulating film 11 an electrostatic charge pattern is observable on insulator 11 after the electrodes have been separated. By using the apparatus of FIG. l in connection with an insulating film 11, comprising a few microns of Staybelite, image resolutions of 47 lines per millimeter have been achieved wit-h storage life times in excess of a minute. For maximum resolution, all film and gap thicknesses should be kept as small as possible. Preferably, the oil or interference film has a thickness which is non-gap- -bridging Under optimum conditions the amount of tiny droplets of oil transferred across the gap is very small, but is suficient to form a visible interference pattern for use in the previously described ways.

While t-he present invention has been particularly described in terms of a specific embodiment thereof, it will be understood that in view of the foregoing specification numerous deviations therefrom and modifications thereupon may be readily devised by those skilled in the art without departing from the present teaching. Accordingly, the present invention is to be broadly construed and limited only by the scope and spirit of the claims now appended hereto.

What is claimed is:

1. The image forming method comprising the steps of applying to the surface of at least one of a photoconductive insulating layer and an electrode a thin liquid film of oil that offers high resistance to current flow said oil having a sufficiently low vapor pressure to form a stable thin film and a wettability below that of the critical value of the surface to which it s applied, said liquid lm because of its thinness exhibiting light interference patterns;

placing said photoconductive insulating layer and said electrode close together but without contact so that said layer and electrode are separated by a gap which does not exceed .001 inch;

applying a voltage of between about 500 to 1200 volts between said photoconductive insulating layer and said electrode; and

exposing said photoconductive insulating layer to an image pattern of light whereby an image is formed by liquid transfer between said electrode and said photoconductive insulating layer. 2. The method in accordance with claim 1 including the step of charging said photoconductive insulating layer prior to the voltage application.

3. The method in accordance with claim 1 wherein said electrode is transparent and said exposing step is carried out through said electrode.

4. The method in accordance with claim 1 wherein said photoconductive layer is supported on a transparent support member and said exposing step is carried out through said support member.

5. The method in accordance with claim 1 wherein said electrode has a solid insulating resin layer on the surface thereof adjacent to said photoconductive insulating layer. 6. The image forming method comprising the steps of: applying to the surface of at least one of a photoconductive insulating layer and an electrode having a solid insulating resin layer on the surface thereof, a thin liquid film of oil having a thickness no greater than two microns which offers a high resistance to current ow, said oil having a sufficiently low vapor pressure to form a stable thin lm and a wettability below that of the critical value of the surface to which it is applied and because of its thinness, exhibiting light interference patterns, placing said photoconductive insulating layer and said electrode, close together but without contact so that said layer and electrode are separated by a gap which does not exceed .001 from the opposing surface,

applying a voltage of between about 500 to 1200 volts between said photoconductive insulating layer and said electrode; and

exposing said photoconductive insulating layer in image configuration whereby an image is formed by liquid transfer between said electrode and said photoconductive insulating layer. 7. A method of forming and erasing an image comprising:

applying to a charged photoconductive insulating layer a thin liquid film of oil that offers high resistance to current flow, said oil having a suciently low vapor pressure to form a stable thin film and a wettability below that of the critical value of the surface to which it is applied, said liquid film because of its thinness exhibiting light interference patterns;

placing said photoconductive insulating layer and said electrode close together but without contact so that said layer and electrode are separated by a gap which does not exceed .001 inch;

applying a voltage of between about 500 to 1200 volts between said photoconductive insulating layer and said electrode:

exposing said photoconductive insulating -layer to an image pattern of light whereby an image is formed by liquid transfer between said electrode and said photoconductive insulating layer; and

erasing said formed image from said electrode and said photoconductive layer by exposure of said photoconductive insulating layer to an actinic source of illumination. f8. A method of forming an image comprising the steps o charging a photoconductive insulating layer;

applying to the surfaces of at least one of said photoconductive insulating layer and an electrode, a thin liquid iilm of oil, said oil having a suciently low vapor pressure to form a stable thin lm and a wettability below that of the critical value of the surface to which it'is applied, said liquid lm exhibiting interference patterns and a high resistance to current Hows;

placing said photoconductive insulating layer and said electrode close together but without contact so that said liquid lm on one surface is separated from the opposing surface by a gap having a thickness no greater than the thickness of said liquid lm;

applying a voltage of between about 500 to 1200 volts between said photoconductive insulating layer and said electrode; and

exposing said photoconductive insulating layer in image conliguration whereby an image pattern is formed by liquid transfer between said electrode and said photoconductive insulating layer.

9. A method of forming an image comprising:

applying to the surfaces of a photoconductive insulating layer and an electrode, a thin liquid flm of oil, said oil having a sufficiently low vapor pressure to form a stable thin film and a wettability below that of the critical value of the surface to which it is applied, said liquid lm exhibiting interference patterns and a high resistance to current flow;

placing said photoconductive insulating layer and said electrode close together but without contact so that said interference liquid lm on one surface is separated from the opposing surface by a gap which does not exceed .001 inch;

applying a voltage of between about 500 to 1200 volts between said photoconductive insulating layer and said electrode;

exposing said photoconductive insulating layer in image conguration whereby an image is formed by liquid transfer between said photoconductive insulating layer and said electrode. 5 10. The method of developing the formed image of claim 9 comprising; dusting said formed image with a nely divided powder.

11. The method of developing the formed image of claim 9 comprising; transferring said formed image to a support surface by rolling said formed image onto said support surface and dusting said support surface with finely divided powder after said rolling step.

12. The method of developing the formed image of claim 9 comprising;

treating a support surface with a substance which reacts chemically with said formed image, and

rolling said formed image onto said treated support surface.

References Cited UNITED STATES PATENTS 

